This invention relates to safety test circuit for a combustion control apparatus. In particular, this invention relates to a safety test circuit of the kind above described which is used for checking the off-failure (defined later) of a transistor (hereinafter referred to as a safety transistor) actuating a safety unit which acts to stop the operation of controlled units such as a solenoid-operated fuel supply valve in the event of system failure and also for checking the on-failure (defined later) of another transistor (hereinafter referred to as a control transistor) actuating a control unit which controls the controlled units such as the solenoid-operated fuel supply valve.
In a conventional combustion control apparatus controlling the combustion of, for example, a boiler or a warm air generator, safety test has been effected from the standpoint of combustion. In the safety test, however, the control transistor has only been checked for such a failure that it is maintained in its onstate when it must be turned off (which failure will be merely referred to hereinafter as on-failure), but the safety transistor has not been checked for such a failure that it is maintained in its off-state when it must be turned on (which failure will be merely referred to hereinafter as off-failure). In such a conventional safety test, additional provision of a timer circuit has been necessary for setting the time required for carrying out the safety test as described later. Further, it has been unable in the case of such a conventional safety test to carry out the safety test during the continuous operation of the combustion control apparatus although it can be done before the apparatus is placed in operation.